The present invention relates generally to techniques for analyzing a rhythm structure of a music audio data set. The present invention is applicable, for example, to apparatus and methods for creating accompaniment progression data by setting, in a performance progression order, accompaniment styles related to an accompaniment performance included in audio data of a music piece. The present invention relates more particularly to a technique for analyzing a music piece audio data set whose accurate beat positions are unknown and thereby identifying an accompaniment style having an accompaniment pattern most similar to an accompaniment performed in the music piece audio data set.
In the field of electronic musical instruments, sequencers, etc., it has been conventional to create in advance accompaniment progression data (also called “accompaniment sequence data”) indicating accompaniment pattern data of which one of accompaniment styles, such as rock, pop and jazz and which one of sections, such as intro, main and ending, classified on an accompaniment-style-by-accompaniment-style basis are to be performed in what order. When a user wants to perform a desired music piece with a musical instrument, he or she selects accompaniment progression data matching the desired music piece and causes automatic accompaniment sounds to be audibly reproduced according to the selected accompaniment progression data by use of an electronic musical instrument or sequencer. Thus, while the user is executing a melody performance of the desired music piece with the musical instrument, automatic accompaniment sounds based on the selected accompaniment progression data can be simultaneously reproduced in response to the progression of the melody, and thus, the user can readily enjoy a musical instrument performance with accompaniment. An example of such a technique is disclosed in Japanese Patent Application Laid-open Publication No. 2000-163064 (hereinafter referred to as “Patent Literature 1”).
Further, recently, a technique has been known which analyzes an audio signal to detect a rhythm pattern and then identifies a reference rhythm pattern most similar to the detected rhythm pattern by performing matching between the detected rhythm pattern and reference rhythm patterns prestored in a database or the like. In an apparatus disclosed in Japanese Patent Application Laid-open Publication No. 2008-275975 (hereinafter referred to as “Patent Literature 2”), for example, beat positions of an audio signal of a music piece are detected, time (or meter) and bar lines are determined on the basis of the detected beat positions, the audio signal is extracted per bar or measure on the basis of the determined bar lines, then a rhythm pattern is detected on the basis of a corresponding one of the extracted measure-by-measure audio signals, and then a reference rhythm pattern matching the detected rhythm pattern is identified. Namely, the prior art apparatus first determines bar lines in accordance with the detected beat positions, then a rhythm pattern is detected per measure based on the detected bar lines, and then performs rhythm pattern matching by comparing the detected rhythm pattern with the prestored reference rhythm patterns.
With the technique disclosed in Patent Literature 1, if accompaniment progression data has been created in advance, the user can, while executing a manual performance of a melody line, cause accompaniment, matching the melody line, to be performed with a simple operation. However, in a case where the user wants to perform manually, together with an automatic accompaniment, a music piece recorded on a newly released CD, for example, the user has to newly create accompaniment progression data matching the new music piece. But, in such a case, it is very likely that the user does not know which section data (hereinafter referred to also as “style data”) of which an accompaniment style should be selected. Even if the user selects a style data set, judging the style data set to be suitable, it often turns out that the selected style data set does not match the music piece to be performed, when the automatic accompaniment is actually executed. Thus, the user has no other choice than to create accompaniment progression data while applying suitable style data sets and confirming how the data sets work on a trial-and-error basis, but such operations tend to be very troublesome and time-consuming.
Therefore, it is conceivable to use the disclosed technique of Patent Literature 2 to analyze a rhythm pattern of the music piece and then create accompaniment progression data on the basis of the rhythm pattern analysis. However, because the disclosed technique of Patent Literature 2 detects positions of bar lines on the basis of beats detected through beat position detection of the audio signal, the detected positions of bar lines are not necessarily accurate. Namely, with the conventionally-known beat position detection scheme, it is difficult to accurately detect each beat of the meter of the music piece, and a half beat, two beats or the like may often be erroneously detected as one beat. In a case where tones or sounds of half notes occur successively over a plurality of measures from the beginning of a music piece of four-four time, for example, a length of a half note may be erroneously recognized as one beat, and a length of two measures may be detected as one measure. Further, in a case where sounds of eighth notes occur successively over a plurality of measures from the beginning of a music piece of four-four time, for example, a length of an eighth note may be erroneously recognized as one beat, and a length of a half measure may be detected as one measure. A rhythm pattern detected in accordance with bar line positions that may be inaccurate cannot necessarily be a rhythm pattern accurately reflecting therein a performance. Therefore, a style data set most similar to a performance pattern performed in the audio signal cannot necessarily be identified through matching between such an inaccurate rhythm pattern and the reference patterns.